The night of 1 May, the date of the translation of Walburga’s relics to Eichstätt in 870, is known as Walpurgisnacht; it is also a pagan festival marking the beginning of summer and the revels of witches. Though the saint had no connection with this festival, her name became associated with witchcraft and country superstitions because of the date. It is possible that the protection of crops ascribed to her, represented by three ears of corn in her icons, may have been transferred to her from Mother Earth and the connection to this pagan holiday.

At present the most famous of the oils of saints is the Oil of Saint Walburga (Walburgis oleum). It flows from the stone slab and the surrounding metal plate on which rest the relics of Saint Walburga in her church in Eichstädt in Bavaria. The fluid is caught in a silver cup, placed beneath the slab for that purpose, and is distributed among the faithful in small vials by the Sisters of Saint Benedict, to whom the church belongs. A chemical analysis has shown that the fluid contains nothing but the ingredients of water. Though the origin of the fluid is probably due to natural causes, the fact that it came in contact with the relics of the saint justifies the practice of using it as a remedy against diseases of the body and the soul. Mention of the oil of Saint Walburga is made as early as the ninth century by her biographer Wolfhard of Herrieden. - from the Catholic Encyclopedia article Oil of Saints